All clear at Washington state LNG plant after unexplained blast: Williams

(Reuters) - A forced evacuation of the area around Williams Cos Inc's damaged liquefied natural gas facility in rural Washington state has been lifted following an explosion on Monday.

What prompted a blast that injured five workers and set off a potentially dangerous leak in a massive LNG tank at the site, near the southeastern Washington town of Plymouth, is still unknown and under investigation, the company said on its website.

"All residents were free to return home. Company personnel were on site to continue to secure and evaluate the operations and safety of the facility, while developing a plan for assessing the damage and the cause," Williams said in a statement posted on its website late Tuesday.

The town of Plymouth, on the Columbia River near the Washington-Oregon border, has around 400 residents.

Earlier, emergency responders had evacuated residents and workers within a 2-mile radius of the LNG storage facility, whose large tanks can store a combined 2.4 billion cubic feet of gas, or enough to meet up to 3.4 percent of daily U.S. natural gas demand.

The evacuation stemmed from concerns that ongoing leaks from one of two 134-foot tall tanks at the site, whose walls were breached by shrapnel from Monday's blast, could prompt a second, stronger explosion.

Michele Swaner, a spokeswoman for Tulsa, Oklahoma-based Williams, said on Wednesday that an investigation still underway showed the explosion on Monday had occurred in or around the site's control room building. The control room contains electrical equipment, she said.

Earlier, emergency responders for Benton County, where the facility is located, had said it was a pipeline within the site that exploded, sending shrapnel into one tank and causing LNG to leak out.

But on Wednesday, responders said they had pinpointed the explosion to a "processing vessel" next to the control building. It was not clear what the vessel contained.

The explosion "did send shrapnel across the site that penetrated the storage tank and caused damage in many locations, including the operations building," said Jeff Ripley, captain at one of the Benton County fire districts.

Joe Lusignan, a spokesman for the Benton County Sheriff's Department said the explosion threw pieces of shrapnel weighing an estimated 250 pounds (113.4 kg) up to 300 yards (meters).

There was no indication of foul play, but Lusignan said the county had "detectives working with the Williams investigators to ensure that we are there if there are indications of foul play".

"Right now we don't have any indications of foul play but we want to cover our bases just in case. Williams has told Benton County officials it may take weeks before the company knows what caused the explosion," Lusignan said.

Monday's explosion injured five workers at the site, one with burns and the others from flying debris.

NORTHWEST PIPELINE REMAINS OPEN

The LNG facility and a compressor station remain shut pending repairs. A major natural gas trunk pipeline in the region, the 3,900-mile-long Williams-operated Northwest Pipeline that supplies several Western states, is operating normally with gas deliveries uninterrupted, the company said.

The Williams site is known as a natural gas peak-shaving facility, which stores gas in super-cooled liquefied form to augment pipeline gas deliveries in times of peak demand.

The tanks had been around one-third full at the time of Monday's explosion. How much gas subsequently leaked out of one tank isn't clear.

The tank, and valves and pipes connected to it developed two separate leaks, Ripley said.

On Monday, leaking LNG froze the ground before evaporating into the atmosphere, Swaner said. Workers then turned the valves to shut off one leak and patched the other. The storage tanks are double walled and a layer of volcanic glass is used as insulation between the walls.

As a pressurized liquid cooled to around minus 260 degrees Fahrenheit, the tanked gas takes up just 1/600th the space it would occupy as a vapor, Williams said on its website.

LNG liquid itself isn't considered explosive. But the gas that boils off when heated can burn if it is mixed with the right amount of air. If gas vapors become trapped in a confined area they can explode.

Several regulatory agencies are involved in the investigation alongside Williams, including the federal Pipeline and Hazardous Materials Safety Administration, Washington Utilities and Transportation Commission and the Federal Energy Regulatory Commission.

(Reporting by Scott DiSavino and Joshua Schneyer in New York Editing by Bernadette Baum and James Dalgleish)